Starting in Elasticsearch 6.2.0, JVM GC logging is enabled out of
the box. This lightweight facility gives insight into GC issues. The settings
for this are in jvm.options and default to keeping 2 GB worth
(rotating every 64 MB) of logs in the default logs directory. If you are
upgrading from a previous version of Elasticsearch and preserve your existing
jvm.options, you should add the following GC logging settings
so that this valuable debugging information is available:

If you have installed Elasticsearch from the RPM or the Debian packaging
distributions and use the default logging location use /var/log/elasticsearch
otherwise if you have installed from the tar.gz or zip archive distributions
and use the default logging location use logs/gc.log; otherwise, set the
location to the absolute path to your log files.